Cracked masonry, or concrete in foundation walls, swimming pools and other structures allows elements such as water and insects to pass from one side of the wall to the other. Placing silicone based caulks in such cracks is unsatisfactory, particularly in long vertical cracks, because such caulks do not bond sufficiently to concrete. The terms silicone rubber, silicone caulk and silicone sealant are used here interchangeably. Commercially available adhesives for silicone caulks have been found unsatisfactory, and many use noxious components, posing safety and environmental problems. One such adhesive is described in U.S. Pat. No. 3,455,762.
Terpolymers are polymers built from three component monomer units. By varying the ratios of monomer units, benefits from each type of unit can be utilized. Such polymers, because each molecule offers a choice of three different functional groups to bond with a surface, are likely to bond to a variety of surfaces. Vinyl acetal polymers are of particular interest. Acetals are formed by reacting alcohols with aldehydes. When the alcohol used is a polyvinyl alcohol, a polyvinyl acetal (or vinyl acetal polymer) is obtained.
Polyvinyl acetate is obtained by polymerization of vinyl acetate. Polyvinyl alcohol is obtained by reacting the polyvinyl acetate with methanol. Polyvinyl acetates are commonly used for adhesives.
U.S. Pat. No. 4,434,257 discloses a cement composition used as either a cement mortar for repair or a prime-coating cement mortar, using an ethylene-vinyl acetate copolymer.
Properties of polyvinyl acetal resins depend on the original ester selected, the extent of hydrolysis, the aldehyde employed and the extent of reaction between the alcoholic and aldehyde groups. Acetals are tougher, more resistant to water, but generally less resistant to weathering than corresponding acetates.
Of interest to the invention is polyvinyl butyral (CAS Registry Number 63148-65-2*), although other acetals might be effective. These resins are practically nontoxic by ingestion (LD.sub.50 &gt;10 gm/kg), single dermal applications (LD.sub.50 &gt;7.94 gm./kg), nonirritating to skins of rabbits and only slightly irritating to the eyes. Polyvinyl butyral is a widely used resin for the interlayer in safety glass, is not discolored by ultraviolet light and retains its toughness even at low temperatures. The replacement of acetate groups with bulky butyl groups changes the properties of the resin, resulting in a more hydrophobic polymer with a higher heat distortion temperature. At the-same time, the polymer's toughness and adhesion to various substrates is considerably increased. Vinyl acetal films are characterized by high resistance to aliphatic hydrocarbons and most oils.
Polyvinyl butryal resins are generally soluble in alcohols, glycol ethers and certain mixtures of polar and non-polar solvents.
When an alcohol is the only solvent, viscosity of a solution tends to increase as the molecular weight of the alcohol increases. Previous literature suggests a blend of alcohols with aromatic solvents to develop a vinyl acetal solvent system. As a result, a higher ratio of resin solids to solvent can be obtained by using a low molecular weight alcohol. It has been noted that when alcohols are employed in a polyvinyl butryal solvent system, that the 99% grade should be used, since the presence of water gives lower solution viscosities. It has also noted that increasing the ratio of resin solids to solvent has little effect on the viscosity of the solution. Likewise, if the number of hydroxyl (--OH) groups in the polymer is insufficient, the polymer can be insoluble in certain alcohols, particularly methanol.
It has been noted that in some solvents, including ethanol and methyl ethyl ketone, polyvinyl butyral resins swell. Another phenomenon affected by solvent choice is "cobwebbing". Cobwebbing can be observed by dipping opposing fingers into resin solution and tapping the fingers together until the solution becomes tacky. In solutions where cobwebbing occurs, many thin strands of resin form.
While the principle use of polyvinyl butyral resins is in lamination of safety glass, others have been suggested. These include overcoating magnet wire (additional crosslinking used), surface coatings, wash primers (anticorrosive metal conditioners), improved metal coatings, wood finishes, knot sealers, printing inks, textile coatings, hot melt adhesives, modification of other resins, and structural adhesives.
Structural adhesives were originally developed for the aircraft industry to replace rivets. Refinements led to use in bonding brake linings, printed circuits, and architectural use for manufacture of interior and exterior curtain walls. These structural adhesives generally blend phenolic and acetal resins. Structural adhesives based on polyvinyl acetal resins can be applied as a solution, an unsupported film, a supported film on paper or cloth, or as a mixture of liquid and solid. If the adhesive is to be brushed on, solvents in the boiling range of 75.degree.-100.degree. C. are advised because they can be removed by air drying and then force dried for 0.5 to 1.0 ours at 105.degree. C.
U.S. Pat. No. 2,261,983 discloses compositions containing polyvinyl acetals in combination with phenol-aldehyde resins for laminations to increase the electrical resistance between laminations of a bonded iron core. The solvent mixture suggested is one third ethyl alcohol.
U.S. Pat. No. 2,403,077, incorporated herein by reference, discloses a method of producing a metal laminated structure. The inventor, Hershberger, notes that ". . . , polymeric vinyl plastic materials such as polyvinyl acetal, polyvinyl acetate, and the like immediately suggest themselves. However, the polymeric vinyl compounds are not self-bonding to metal, and the lack of a suitable adhesive for bonding these plastic compounds to metal has constituted a serious obstacle to their wide use either as protective coatings or for fabricating plastic to metal structures." The "polyvinyl butryal" used in the resin contained a phenolic-formaldehyde component, in particular, a polyhydric-formaldehyde group.
U.S. Pat. No. 2,499,134, incorporated herein by reference, discloses a structural adhesive system using a phenolic adhesive sprinkled with a polyvinyl acetal. The sections to be joined were then brought together and heated at temperatures at temperatures above 200.degree. F. (93.3.degree. C.).
Silicon caulking, glazing and sealing compounds are made with a variety of characteristics, and for purposes of this application will be collectively referred to as silicone sealants. Properly bonded, silicone sealants are capable of up to .+-.50% joint movement. Because acetoxy cure silicone sealants emit fumes upon curing, and lack appropriate environmental resistance, they are not appropriate for use on interior cracks. RTV or room temperature vulcanizing sealants are available as odorless, neutral-cure sealants formulated to be non-corrosive to most building materials. However, when applied to large vertical cracks in cement, these peel off, therefore being unsatisfactory for this application.
The subject invention of this application fills a need for a low toxicity adhesive, capable of bonding a variety of materials, including silicone sealants, which can, in particular, be used to seal cracks, especially large vertical cracks, in foundations.